A typical manhole assembly comprises a frame including an upstanding wall having protruding outwardly therefrom a peripheral flange. The flange usually extends from the lowermost edge of the wall and on installation of the manhole assembly is buried in a bedding medium such as mortar in order to secure the frame over au aperture such as the entrance to an inspection chamber or drain. Thus the upstanding wall in practice defines the boundary of the aperture, which boundary is in most cases rectangular although in some cases it is circular.
It has been found to be particularly convenient to use triangular cover plates, that are receivable on seatings forming part of the upstanding wall, in order to close a rectangular aperture of the aforesaid type.
The cover plates typically are removable from their position closing off the aperture.
The rectangular frames usually are either in a form defining a substantially square aperture; or in a form deeming an elongate, rectangular aperture.
In the former case a single pair of triangular cover plates is sufficient to close off the aperture. In the latter case typically two pairs of the cover plates are used.
The only way of occupying a generally square aperture through the use of two triangular cover plates is to employ right-angled triangular shapes for the plates. Such shapes are also suitable for use in sets of pairs for closing off the elongate rectangular apertures described hereinabove. Triangular plates are particularly appropriate since they readily lend themselves to being supported on their undersides at three locations corresponding approximately to the apices of the triangular shapes. This renders the cover plates largely non-rocking, and hence generally silent in use, even when for example road traffic has caused substantial wear of the seatings on which the apices of the triangular cover plates rest.
Despite these well-established advantages of triangular cover plates, several problems remain.
One of these derives from the mass of the cover plates.
Typically the plates are cast from iron. Consequently they weigh several tens of kilograms. A worker attempting to lift a cover plate free of the aperture defined by the manhole frame may consequently suffer strain injuries, especially if his lifting technique is defective; or impact injuries caused by incorrect handling (ie. dropping etc) of the plates.
Partly in response to this problem, there have been numerous proposals for hingedly securing the triangular cover plates to the upstanding wall of the frame.
This provides a partial solution to the problem of potential injury, since firstly the worker does not have to lift the entire cover plate mass (some of which is supported by the hinges); and secondly there is a reduced, but not entirely eliminated, chance of dropping a cover plate so as to cause injury.
However, several disadvantages of hingedly secured cover plates have become apparent.
One of these derives from the manner in which a worker typically opens the cover plates.
Each triangular cover plate is hingedly secured along a single, non-hypotenuse side, to the frame. Thus a given pair of the cover plates are juxtaposed along their respective hypotenuses when closed in order to cap the aperture.
A worker wishing to open such a cooper plate typically, would stand outside the frame with his feet adjacent such a non-hypotenuse side. He would then lean forwardly for the purpose of inserting a lifting tool into an aperture formed adjacent the apex of the cover plate furthest from him.
A commonly used lifting tool includes a gripping handle by means of which, following its insertion into the aperture, the worker is able to draw the cover plate in an upwardly extending arc utilising the hinges.
Necessarily this action involves the worker in stretching forwardly in order firstly to insert the lifting tool and secondly to draw the cover plate upwardly.
Physiotherapists and ergonomists have for long recognised that lifting in the resulting semi-squatting position can, for some users, cause difficulties. Therefore it would be desirable to eliminate this mode of lifting of cover plates.
Furthermore, cover plates that are hingedly secured to the manhole frames typically include hinges that are releasable when the cover plates are in their open positions.
This allows the complete removal of each cover plate from the frame.
However, lifting of the cover plates from their open positions clear of the frame may be a sub-optimal lifting method since the worker may attempt to grip the cover plate adjacent its free apex. The tapering form presented at this location may be difficult to grip, thereby increasing the difficulty of lifting.
In addition to the foregoing, the known pairs of right-angled triangular cover plates are of equal sizes. If one of them is removed from the frame as aforesaid, it becomes possible on attempting to remove the second of them to drop the second cover into the aperture bounded by the frame, with potentially troublesome results.
Even if this does not happen the step of opening one of a pair of generally identical triangular cover plates results in opening of 50% of the aperture area that they cover when closed. The resulting opening may be large enough to allow eg. tools to fall into the aperture.